This invention relates to methods and apparatus for recovering gases, principally hydrocarbon gases, from geopressured, hot salt water, and in particular relates to methods and apparatus for recovering hydrocarbon gases dissolved in geopressured, hot salt water by substantially reducing pressure on the hot salt water and trapping the released gases. In addition, establishing an electric current flow through the salt water aids in the disassociation of additional hydrocarbon gases from the water. Additionally, method and apparatus are provided for recovering thermal energy from the hot salt water for conversion into electrical and mechanical energy.
There is a known sub-surface belt roughly 200 miles wide stretching from New Orleans, La. to Brownsville, Tex., which has underground formations containing geothermal salt water in which is dissolved large quantities of hydrocarbon gases. The salt water is trapped at depths of 10,000 feet or greater, and the great pressure in the earth's crust at those depths have caused large quantities of hydrocarbon gases to be dissolved in the salt water. Additionally, the pressures at those depths have caused an elevation in the temperature of the water ranging upwards of 300.degree. F.
A U.S. geological study estimated that the geothermal hot salt water under Texas and Louisiana contains 24,000 quads of methane gas within drilling range, but some experts say that there is more than that. A "quad" is a macro-unit of measurement that represents one quadrillion British Thermal Units (BTU). A BTU is the amount of heat it takes to raise the temperature of a pound of 39.degree. F. water 1.degree. Fahrenheit. Thus, a quad is roughly equal to the BTU's in 170 million barrels of oil, or a trillion cubic feet of natural gas.
The geopressured area has been precisely defined by the more than 300,000 oil and gas exploration boreholes which have been drilled in Texas and Louisiana since 1920. Additionally, and before instruments were perfected for early detection, hitting the geothermal area unexpectedly was disastrous in drilling an oil or gas well. Sudden entry into subterranean strata and encountering pressures of 11,000 pounds per square inch or more often resulted in a violent blow-out, completely clearing the borehole of drill stem and casing.
There still are some problems, since engineers and geologists still do not know exactly how much gas there is, or exactly at what depth it is, or whether the water everywhere within the belt in saturated with gas, or whether some water has just small amounts of gas. Further, geologists and engineers do not know the temperature of the water in all areas, nor how saline the water is, and whether the salinity changes from place to place. In saturated salt water, gas content has been found to range from 6.7% to 11%. At a recent geothermal well under test in Louisiana, the water temperature, when brought to the surface was 220.degree. F., 8.degree. above the boiling point. The temperature is important because, in addition to gas, thermal energy may be retrieved from the hot water for use in other ways to produce energy for industrial application. For instance, heat diverted into freon boilers could turn turbines and thus generate electricity. Additionally, the salt water must be reinjected into the formation in order to maintain the geopressure therein to prevent land subsidence caused by the removal of the pressurized water and to meet environmental restrictions on the disposal of salt water.
The prior art technology presently being utilized requires that the hot geopressured salt water be first introduced into a first pressurized tank that would allow some of the dissolved natural gas to be freed by lowering the pressure from approximately 11,000 psi to about 6,000 psi. Then the pressure in the tank would be lowered again to about 3,000 psi to allow more gas to evolve until the largest percentage of gases can be obtained, and then the high temperature water can either be reinjected into the formation or applied to suitable means for converting the heat into electrical or mechanical energy and then the water would be reinjected into the subsurface formation.
One disadvantage of the fixed containers utilized in the prior art is the large margin of safety which must be built into the containers to accommodate overpressurization resulting from large, sudden increases in pressure as often occur when a large high-pressure "slug" of gas is produced. A second disadvantage of the prior art is the complex apparatus required to permit successive reductions in the pressure without losing the gases produced by the initial reduction in pressure.
The present invention overcomes the deficiencies of the prior art by providing methods and apparatus for permitting a direct reduction to atmospheric pressure of the geopressured hot salt water, thereby maximizing the release of hydrocarbon gases contained therein.